1. Field of the Invention
The invention relates generally to a thyristor and more particularly to a thyristor which can be switched by an incident light beam. Further, the invention relates to a method of producing such a thyristor.
2. Description of the Prior Art
The firing of high-power thyristors requires considerable external circuitry, expecially for a series connection of several thyritors, as for example in larger converter systems, such as HGUe-rectifiers. It is known how to reduce such circuit requirements by firing the thyristor nonelectrically, e.g. by illumination with a light beam (cf. e.g. U.S. Pat. No. 3,697,833). The known thyristor structures have the disadvantage, however, that the available light energy is not used in the optimum manner. In silicon, moreover the strong absorption at wavelengths below 0.51 microns has a very adverse effect. On the other hand the light energy must be at least 1.11 eV (.lambda.=1.1 microns) in order to produce electron-hole pairs. The absorption coefficient of silicon, however, becomes greater than 10.sup.4 cm.sup.-.sup.1 beyond 2.43 eV (.lambda.=0.51 microns), so that the light only penetrates 1 micron into the semiconductor structure. To increase this value one could of course use infrared light sources of high intensity, e.g. Nd:YAG laser with .lambda.=0.89 microns. However, the use of such lasers is rather expensive. A further possibility for firing with light, the indirect method in which a light-sensitive, current amplifying element is illuminated, requires a high switching-power consumption according to present conceptions, since the firing current must be obtained from the main circuit while the voltage at the light-sensitive element must be held constant, even if the forward voltage varies over three orders of magnitude.